Surface treatment of aluminum and aluminum alloys



Patented Jan. 25, 1944 SURFACE TREATMENT OF ALUMINUM AND Q ALUMINUM ALLOYS Alfred Norman Douglas Pullen,

London, England,

assignor, by mesne assignments, to Aluminum Company of America, ration of Pennsylvania Pittsburgh, Pa., a corpo- No Drawing. Application March 31, 1939, Serial No. 265,339. In Great Britain Aprll 7, 1938 2 Claims. (Cl. 204-140) This invention relates to the surface treatment of aluminum and aluminum alloys and more especially to treatment directed to the improvement of the reflectivity of such surfaces.

The object of the present invention is the provision of a method and means by which the reflectivity of an aluminum or aluminum alloy surface may be increased very considerably in a cheap, efficient and permanent manner.

In my prior Patent Specification No. 2,096,309 I have described a process for increasing the refiectivity of the surface of an aluminum or aluminum alloy article which comprises immersing the previously degreased article in an aqueous solution of sodium carbonate and sodium orthophosphate to remove the normal oxide film, then in the same solution subjecting the article to anodic treatment by direct current at a moderately high current density for a short time to stop the attack of the solution upon the article and continuing the anodic treatment by direct current at a lower current density, and, after washing in water subjecting the article to further electrolytic treatment in a strong aqueous solution of sodium bisulphate.

I have found, as a result of careful and extended research, that beneficial results are obtained by modifying the composition of the first i. e. the alkaline bath in the above described process. These beneficial results include a re-' duction of the current density, including current surges (up to 30 to 40% in some cases) and a less pronounced preliminary etch, giving a sharper reflected image and less prominence to any structural and other markings which may exist on the surface of the metal.

The present invention consists in a process for improving the reflectivity of the surface of -a clean aluminum or aluminum alloy surface which comprises immersing the surface for a short time in an electrolyte formed as an aqueous solution containing sodium carbonate, with or without additions of sodium tribasic orthophosphate, and containing additions of ammonia, ammonium carbonate, ammonium phosphate, ammonium molybdate or other ammonium salts, and having av pH value of or more and a temperature of about 75 to 85 C. and then without removing the surface from the solution applying direct current anodic treatment thereto.

Several ways of carrying the invention into effect will now be described by way of example.

Example I A piece of. commercially pure rolled aluminum sheet'is first degreased by suitable means such as cleaning in petrol, then, if necessary, it is buffed or otherwise polished mechanically. It is then given a chemico-electrical treatment in a bath having the following composition:

15% anhydrous sodium carbonate. 6% crystalline tribasic sodium orthophosphate. 1% ammonium carbonate.

The piece of sheet is immersed in the bath with the result that sol tion of the metal by the alkaline electrolyte commences almost immediately. As soon as uniform attack is observed direct current at 10-14 volts is switched on, the aluminum being the anode, the current density applied being of the order of 10 to 15 amperes per square foot. An iron or steel cathode is used and procedure in the bath follows electro-plating practice to some extent.

It is of advantage to have a bath which is of considerably larger dimensions than the electrodes. At the end of a short period, not more than 20 seconds the current density automatically falls to about half the first valuewhilst the voltage is maintained approximately at the figure mentioned. The treatment is continued under these conditions for about 10 minutes. The article is then removed from the bath, preferably with the current still on, and immediately washed in clean water. The surface of the metal has now a very bright appearance and it will be found that if the original reflectivity value was, say, it has now been raised to perhaps Besides the general bright appearance, it will be noticed that the surface of the metal now carries a thin film, presumably of aluminum oxide, which shows brilliant interference colours.

Example II The electrolyte has the following composition:

15% anhydrous sodium carbonate 2% ammonium orthophosphate and the procedure follows that of Example I.

In all the above examples the percentages are by weight, the solution is used at a temperature of about 80 C. and the treatment may be followed by treatment in a bisulphate bath for the provision of a protective film as described in detail in specification No. 2,096,309.

General Although temperatures of about 75 to are preferable for the alkaline bath, good results under certain conditions may be obtained over the temperature range of 55 to C.

2 a f aasaaoo n is important that the pH value 0! the alkaline solution should be 10 or more, due to the presence of carbonates or hydroxides or both, the

ercentages being from about 3 to about 20% by weight of sodium carbonate or irom about 0.1 to about 3.0% by weight of sodium or potassium hydroxide, the specified additive materials in either case, such as phosphates oi the alkali metals. being from about 0.5 to about 5.0% by weight. It will be noted that the concentration of the ammonium salt used in Examples 1 and 2 ranges from 1 to 2%, this being an advantageous range.

I claim:

1. In a process for improving the reflectivity or aluminum or aluminum alloy surfaces and producing a surface with decreased prominence of structural and other markings, the step which comprises electrolytic brightening of said surface by subjecting it to direct current anodic treatment in an aqueous electrolyte containing about 3-20 per cent sodium carbonate together with about 0.5-5 per cent of an ammonium salt, said electrolyte having a pH value of at least about 10.

2. In a process for improving the reflectivity 0! aluminum or aluminum alloy surfaces and producing a surface with decreased prominence oi l0. structural and other markings, the step which comprises electrolytic brightening oi said surface by subjecting it to direct current anodic treatment in an aqueous electrolyte containing about 3-20 per cent sodium carbonate and about 0.5-5

1| per cent sodium phosphate together with about 0.5-5 per cent of an ammonium salt, said electrolyte having a pH value of at least about 10.

ALFRED NORMAN DOUGLAS m. 

